Version #1

This version was discovered in an article on http://kevin.vanzonneveld.net.

function number_format( number, decimals, dec_point, thousands_sep ) {
    // http://kevin.vanzonneveld.net
    // +   original by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +     bugfix by: Michael White (http://getsprink.com)
    // +     bugfix by: Benjamin Lupton
    // +     bugfix by: Allan Jensen (http://www.winternet.no)
    // +    revised by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
    // +     bugfix by: Howard Yeend
    // *     example 1: number_format(1234.5678, 2, '.', '');
    // *     returns 1: 1234.57     
 
    var n = number, c = isNaN(decimals = Math.abs(decimals)) ? 2 : decimals;
    var d = dec_point == undefined ? "." : dec_point;
    var t = thousands_sep == undefined ? "," : thousands_sep, s = n < 0 ? "-" : "";
    var i = parseInt(n = Math.abs(+n || 0).toFixed(c)) + "", j = (j = i.length) > 3 ? j % 3 : 0;
    
    return s + (j ? i.substr(0, j) + t : "") + i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + t) + (c ? d + Math.abs(n - i).toFixed(c).slice(2) : "");
}

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Version #2

Based on my baseline format_number function, and stored on github.

function format_number(n,prec,dec,sep) {
    // Original by Luke Smith (http://lucassmith.name)

    n    = !isFinite(+n) ? 0 : +n;
    prec = !isFinite(+prec) ? 2 : Math.abs(prec);
    sep  = sep == undefined ? ',' : sep;

    var s   = n.toFixed(prec),
        abs = Math.abs(n).toFixed(prec),
        _, i;

    if (abs > 1000) {
        _  = abs.split(/\D/);
        i  = _[0].length % 3 || 3;

        _[0] = s.slice(0,i + (n < 0)) +
              _[0].slice(i).replace(/(\d{3})/g, sep+'$1');

        s = _.join(dec || '.');
    }

    return s;
}

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